Significant research effort is currently underway to develop fuel reformer technology and applications. A fuel reformer mixes liquid fuels with a controlled quantity of air and cracks the fuel into a mixture of gases generally comprising hydrogen (H2), carbon monoxide (CO), and small amounts of carbon dioxide (CO2), water (H2O), and methane (CH4). This mixture of gases is termed “reformate.” Reformate provides a clean fuel that can be used in other energy conversion devices such as internal combustion engines or fuel cells such as solid oxide fuel cells. Reference, for example, U.S. Pat. Nos. 6,230,494, 6,655,325, 6,609,582, and 6,485,852, the disclosures of each of which are totally incorporated by reference herein. Alternatively, reformate can be used as an improvised source of reducing gas for other pollutant treatment systems such as, for example, nitrogen oxides (NOx) absorber regeneration systems. A fuel reformer, together with a water shift reactor, can further enrich H2 concentration by converting CO and H2O into H2 and CO2. Such a system is the source of hydrogen fuel for hydrogen burning engines or proton exchange member (PEM) fuel cells.
It is desirable to control the oxygen to carbon (O/C) ratio of the air-fuel (A/F) mixtures supplied to the reformer. Ideally, one would like to maintain an O/C ratio of unity, providing just enough oxygen to crack hydrocarbon liquid fuel in H2 and CO with minimum production of CO2 and H2O. However, a stoichiometric supply of oxygen would, inadvertently, raise the reformer temperature to a point where the reformer catalyst can be thermally damaged, for example, above about 1000° C. for precious metal or non-precious metal type catalysts. An inadequate O/C ratio together with low temperature, for example, temperature in the range of below about 800° C. to about 550° C., can form carbons also, which can poison the reformer catalyst as well as poison down stream fuel cell electrodes in those systems wherein the reformer is providing fuel stock to a fuel cell. Further, the release of soot can pollute the environment.
In bench testing, it has been shown that it is possible to monitor reformate O/C ratio using a mass spectrometer. However, such an approach is not practical for real world applications.
Sensors, for example A/F ratio sensors, are known. Exhaust gas sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust sensors have been used for many years in automobiles to sense the presence of selected exhaust gases. Traditional A/F ratio sensors use air or oxygen as a reference gas. In automotive applications, the direct relationship between various exhaust gas concentrations and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the sensor or sensors to provide concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions. Reference, for example, U.S. Pat. Nos. 5,369,956, 6,295,809, 6,532,736, 6,579,435, 6,616,820, 6,746,584, 6,797,138, the disclosures of each of which are totally incorporated by reference herein.
Most sensors have outputs that are functions of temperature. The sensor output will vary as sensor temperature varies. To solve this problem, the sensor can be operated at a constant temperature, or the sensor is operated at a variable temperature which is measured in order to correct for the effect of temperature variation on sensor signal outputs. In either case, a temperature sensor and typically a heater, is built in to the sensing device.
When accurate measurement of the primary sensor is required, the precision of temperature (or associated controls) can present a challenge, particularly, for example, in exhaust gas species sensing applications. As air quality legislation in both Europe and North America become tighter and tighter, current sensor performance must be adjusted to meet the demands. Requirements include precision along with high temperature durability and poison resistance.
Exhaust sensors using zirconia electrolyte impedance as the temperature indicator are known. Zirconia impedance is exponentially dependant on temperature, becoming smaller at higher temperature, which makes it unsuitable for high temperature sensing. Further, the high non-linearity of zirconia at higher temperatures adds complexity to the control algorithm.
Resistance Temperature Detector (RTD) technology is disclosed, for example, in Published U.S. patent application Ser. No. 10/004,679 (Document Number 20030101573A1) assigned to the present Assignee, the disclosure of which is totally incorporated by reference herein. Linear RTD type temperature sensors have been incorporated with other sensing devices. Linear RTD sensors-use a thick film, multi-layer architecture. Typically, the RTD is screen printed using gold (Au) lines because with gold it is possible to achieve a high resistance value in a small area. Due to the low melting points of gold or gold alloys, this approach is not suitable for high temperature exhaust applications. Platinum (Pt) can sustain high temperatures such as experienced in combustion exhaust applications. However, the high conductivity of platinum renders screen printing fabrication approach difficult as it is difficult to achieve a high resistance value in a small area as required by RTD applications.
The disclosures of each of the foregoing U.S. patents are each totally incorporated herein by reference in their entireties. The appropriate components and process aspects of the each of the foregoing U.S. patents may be selected for the present disclosure in embodiments thereof.
What is needed is a practical, cost effective, and easy to manufacture device and method for monitoring and controlling the O/C ratio of an air fuel mixture feeding a fuel reformer.